The present invention relates a cutting insert for a percussion drill bit.
In general, there are four different face designs used in the hammer bit industry. They are: drop center, concave, flat face and convex (sometimes called double bevel or double gauge). Each has its own advantages and disadvantages for different applications. The placement, diameter, shape and number of tungsten carbide inserts used on these bits vary by face design and manufacturer. In general, the smaller the carbide diameter, the faster the penetration rate. The trade-off with the smaller carbides is that they wear out faster and require more frequent re-sharpening. Larger carbides, while giving up penetration rate, will last longer and are less prone to shear failure.
Most face designs incorporate large diameter carbides on the gauge row for greater wear resistance and smaller diameter carbides on the inner rows where wear resistance is minimized as a result of reduced speed of the inserts closer to the longitudinal axis of the bit. The most popular carbide designs are dome-shaped and conical-shaped. Dome-shaped carbides are standard on most bits and conical carbides can be ordered as optional equipment. The advantages of conical carbides are higher penetration rate, bigger chip size and more efficient rock breakage. The primary disadvantage of conical carbides is that they are more fragile than dome-shaped carbides and are therefore used primarily in soft consolidated formations only.
DTH (down the hole) carbides are subject to various types of wear during drilling. The majority of the wear is caused by abrasion as the bit rotates against the bottom of the hole and against the hole wall, causing xe2x80x9cwear flats.xe2x80x9d If wear becomes excessive, bit life and hammer performance can deteriorate. When a carbide is excessively worn, stress is higher on the carbide and can lead to premature failure.
The two types of wear addressed by the present invention are referred to as xe2x80x9cfrontal wearxe2x80x9d and xe2x80x9cgauge wear.xe2x80x9d Frontal wear occurs when drilling in hard rock, such as granite. In this case, the gauge row carbides will wear faster than front carbides because of the greater distance covered around the outside of the bit during rotation. Gauge wear occurs when drilling in abrasive rock with a high quartz content. An xe2x80x9canti-taperxe2x80x9d develops which diminishes the clearance of the bit body caused by unusually high wear to the gauge row. The typical solutions to these problems are: changing the grade of carbide used in the gauge row; increasing the diameter used in the gauge row; re-sharpening the inserts; or decreasing the penetration rate.
Accordingly, it is desirable to provide an improved gauge row cutting insert with extended service life without loss of penetration rate.
The present invention overcomes the above-referenced shortcomings of prior art cutting inserts by providing a cutting insert having the service life expectancy of a dome-shaped insert and the rate of penetration (ROP) of a cone-shaped insert. The insert has a cutting head which is a non-symmetrical dome shape, wherein carbide is reduced in the trailing portion and increased in the working portion of the cutting head.
More specifically, the present invention provides a cutting insert for a gauge row (or other location) of a percussion drill bit, including a generally cylindrical mounting portion extending along a central axis and a cutting head extending from the mounting portion. The cutting head has an exposed surface forming a non-symmetrical dome shape such that a working portion of the cutting head is enlarged with respect to a standard dome shape and a trailing portion of the cutting head is reduced with respect to a standard dome shape. The cutting head has an inner edge and an outer edge. The exposed surface is non-symmetrical along a central plane which bisects the working portion and trailing portion. The exposed surface is symmetrical in a second plane perpendicular to the central plane such that the exposed surface forms an arc of constant radius from the inner edge to the outer edge.
Preferably, the exposed surface is symmetrical in any intersecting plane which is perpendicular to the central plane such that the exposed surface forms an arc of constant radius in any such plane from the inner edge to the outer edge.
The invention also provides a percussion drill bit including a shaft with a boring head positioned at a distal end of the shaft and having a longitudinal axis. The boring head has an upwardly facing upper end with a plurality of holes formed therein. The holes include a gauge row of holes. A cutting insert, as described above, is positioned in each hole of the gauge row of holes.
Accordingly, an object of the invention is to provide an improved cutting insert for a percussion drill bit wherein the cutting insert has an exposed surface which is non-symmetrical in a plane bisecting the working portion and trailing portion, and symmetrical in a second plane perpendicular to the central plane such that the exposed surface forms an arc of constant radius from the inner edge to the outer edge in the second plane.
Another object of the invention is to provide a cutting insert for a gauge row of a percussion drill bit having a cutting head with an exposed surface which is non-symmetrical along a central plane bisecting the working portion and trailing portion, and symmetrical in any intersecting plane which is perpendicular to the central plane such that the exposed surface forms an arc of constant radius in any such plane from the inner edge to the outer edge.